The technology behind self-driving semi-trucks is consistent with that of other self-driving vehicles, however on a larger and heavier scale. A combination of sensors (LiDAR), radar, and cameras all help illustrate the surrounding area in a digital map, whereas the computer can use algorithms to help make decisions.
Currently, the semi-trucks themselves are simply the same general design format (53' truck/trailer) as human operated vehicles, with the key component being the software, cameras, sensors, and radar provided.